Fuel cells provide a low- or no-pollution technology to electrochemically generate electricity at very high efficiencies using a variety of fuels. Use of hydrogen produced from renewable sources as fuel or direct use of biomass-derived fuels in fuel cells for transportation and power generation can contribute significantly to a truly sustainable global economy. In addition, fuel cell-based technologies that include carbon dioxide capture and sequestration are being developed to produce electricity with zero greenhouse gas emissions even with the use of hydrocarbon fuels. Several types of fuel cells have been under development in the last three decades for clean and efficient power generation; most prominent among these are the proton exchange membrane (PEM) and the solid oxide (SOFC) fuel cells. This presentation reviews the materials and performance of these two types of fuel cells and discusses the differences between them, particularly with respect to the fuel requirements. Status of these two types of fuel cells and key challenges associated with materials and electrochemistry are discussed, and directions for future research and development are presented.